1. Field of the Invention
The present invention relates to residual fuel oil conditioners and their use in improving combustion and preventing, inhibiting or removing combustion deposits and corrosion resulting from the burning of residual fuel oils.
Residual fuel oils, such as No. 5 and No. 6 fuel oils, are one of the major fuels used in firing large industrial and institutional boilers. Residual oils are derived from various crudes, for example naphthenic, paraffinic, and Mid-Continent crudes, and they have boiling ranges above 850.degree. F., are liquid at room temperature, and have API gravities of about 1 to 15 or more. The residual oils are attractive economically, being cheaper than other oils, but they pose a serious problem: they contain a higher proportion of various inorganic elements and compounds which result in unwanted deposits and corrosion when the residual fuel oil is burned.
Deposits resulting from combustion of residual fuel oils, referred to as fireside deposits, for example slag, are the result of inorganic contaminants in the fuel. In the high temperature zone of the typical boiler system, for example the waterwalls, screen tubes, superheaters and convection risers, such fireside deposits create a serious problem, ultimately resulting in an unacceptable lowering of heat transfer efficiency.
A particular problem created by combustion of residual fuel oils arises from the concentration of vanadium compounds in such oils. Vanadium not only forms a part of the ash and slag of the fireside deposits, with attendant reduction in operating efficiency of the boiler system, but the vanadium-containing ash deposits also present a serious problem of corrosion.
Upon combustion, complex organic compounds of vanadium, sodium, and sulfur form low melting ash or slag deposits on the firebox, superheater and reheater tubes, supports, hangers, and spacers of a typical boiler. The actual location of ash or slag build-up depends upon the particular boiler design, and the amount of fouling is a function of the oil composition. For example, fuel oils having low sulfur and low vanadium content cause very little fouling, in the high temperature zone, while extensive fouling occurs when the sulfur content is from 2.3% to 3% and the vanadium content is from 300 to 500 parts per million. Since the oxides of vanadium have relatively low melting points, the ash derived from these oxides may be in a plastic state while being carried in the hot combustion gases. When this ash strikes the cooler metallic surfaces of the components of the fuel burning equipment, it adheres tightly. The deposits thus created insulate the metallic surfaces, impede heat transfer and raise the temperature of the outer metallic component surface. Moreover, this condition tends to trap additional ash which might not adhere under normal circumstances to clean metallic surfaces. As gas passages thus become smaller, the velocity, and hence the impingement force of the gases and ash particles increases, and the fouling rate is thereby accelerated. Because of this resultant heat barrier, output of the fuel burning equipment, for example a boiler, can be maintained only at the expense of increased energy input requiring consumption of additional fuel. The result is a less efficient and, consequently, more expensive operation of the fuel burning equipment. Moreover, removal of these slag deposits is very difficult due to their extreme hardness and tight adherence to the metallic surfaces of the fuel burning equipment; and the nature of the equipment itself, particularly modern boilers, makes many parts thereof inaccessible to cleaning.
The oxides of vanadium which produce slag deposits as described above, have also been found to be highly corrosive to metals. For example, vanadium pentoxide and sodium sulfate, both of which are formed during the combustion of residual fuel oils, react to form the most corrosive vanadium slag, .beta.-sodium vanadyl vanadate, in accordance with the following reaction: ##STR1## At 850.degree. C. this vanadate is a reddish colored corrosive liquid which can adsorb oxygen, and when it solidifies it releases this adsorbed oxygen. The resulting slag is a very hard, blackish colored material. Another vanadium slag commonly found in fuel burning equipment such as boilers is sodium vanadate, Na.sub.2 O.2V.sub.2 O.sub.5. However, the present invention is applicable to the problem of corrosion and slag deposits caused by all compositions formed from vanadium, vanadium and sodium, and sulfur, as well as other inorganic and metallo-organic compounds, during combustion of residual fuel oils.
Theories as to the precise mechanism of corrosive attack by vanadium oxide slags on steels vary. The vanadium oxide slags are characterized by low melting points and they are capable, in that state, of dissolving or absorbing oxygen which is then transferred to the metallic surfaces of the fuel burning equipment, ultimately resulting in oxidation, and thus corrosion, of the metal component. An alternative, or concomitant, mode of corrosive attack on steel surfaces by vanadium oxide slags is found in their continuous removal of the normally protective oxide layer from the surface of the steel component.
Unfortunately, the inorganic contaminants in residual fuel oils which create the problems described above are present in such small quantities and their chemical makeup is such that methods for their removal from residual oils are difficult to apply economically on a commercial scale.
Yet another problem created by combustion of residual fuel oils occurs in the cold-end zone of the typical boiler system, for example the economizer tubes, air heaters, fans and stacks, where sulfur trioxide formation and sulfuric acid condensation cause serious corrosion problems. It is generally considered that vanadium oxide deposits effectively catalyze the oxidation of sulfur dioxide contained in the waste gas from typical residual fuel oil burning. The resulting sulfur trioxide combines with water vapor also typically present to form sulfuric acid. This sulfuric acid, upon condensation, can be a source of corrosive attack on the steel components of burning equipment, particularly those portions of such equipment located somewhat downstream from the site of burning. The present invention is useful in preventing corrosive attack upon the steel components of burning equipment by condensed sulfuric acid resulting from reaction of sulfur trioxide and water vapor. The metals of the present invention are multi-functional in their ability to reduce sulfuric acid corrosion and acid-induced deposition in the cold temperature zone. The metals reduce the iron oxide surface which causes catalytic formation of sulfur trioxide, by forming a protective shield over the iron oxide. Further, the combustion improvement capabilities of the metals of the present invention reduce the concentration of unburned carbon, whereby it is thus removed from the sticky sulfuric acid/unburned carbon system. In this particular additional aspect of the present invention, the metal salt aqueous solution conditioners of the present invention, when utilized in the operation of fuel burning equipment, form a protective coating or deposit upon the surfaces of the steel components of the fuel burning equipment, thereby insulating such surfaces from attack by the condensed sulfuric acid. Such corrosive attack by condensed sulfuric acid is most likely to occur in the lower temperature portions of the fuel burning equipment downstream from the site of burning. Thus, the present invention is also effective in preventing corrosion of the steel components of fuel burning equipment caused by sulfur compounds contained in residual fuel oil burned therein. Whether these modes of corrosive attack are found to be operating together, or individually, or whether some other theoretical or proven mode of corrosive attack is considered to be operating, the present invention is not limited thereto, but rather is limited only as claimed herein.
All of the problems described above can be prevented or rendered less serious by the addition to the residual fuel oil, of small amounts of any one or a combination of such metals as magnesium, manganese, zinc, copper, lead, iron, nickel, aluminum, calcium and barium. The different metals contribute in different ways, extents, and degrees to preventing, decreasing, or removing the various deposit and corrosion problems described above, as is known in the art. Thus, the art has focused on different techniques for introducing the metals for treating residual fuel oils into those oils.
2. Brief Description of the Prior Art
Heretofore, basically three approaches have been taken to the problem of how to introduce small amounts of metals or metal salts into residual oils and maintain them in a dispersed state therein for the purpose or preventing, inhibiting or removing deposits and corrosion when the residual oil is burned. First, organic soluble solutions of the metals have been prepared using metalloorganic compounds. While these solutions are easily added to residual fuel oils and are readily maintained in a dispersed state therein, their cost is unacceptably high. Second, oil suspensions of various metal oxides have been prepared, but these are added to the pressurized, heated oil just prior to atomization of the fuel. While these products are relatively inexpensive, they are difficult to feed to the residual oil, and they experience settling on storage. Third, water-in-oil emulsions of various water soluble metal salts have been used for treating residual oils. While these products are cost effective and easy to use, they often experience problems with phase separation. Unlike these approaches of the past, the present invention provides a novel and more efficient residual fuel oil conditioner based on an aqueous solution of the treating metal salts.
The following are referred to for a more detailed description of the deposit and corrosion problems discussed above, as well as some of the solutions which have been explored in the past: U.S. Pat. Nos. 2,845,338; 3,000,710; Canadian Patent No. 967,755; and Japanese Patent Publication No. 12,083 (1960).